Steady and transient shear flows have been used to study the nonlinear behavior of aqueous, very concentrated suspensions of plate-like particles interacting mainly through excluded volume and electrostatic interactions. These interactions lead to an ordered structure corresponding to regions of aligned plates with very small interparticle gaps, as shown by cryomicroscopy. The transient shear behavior, observed at different shear rates, in terms of the shear stress τ and the first normal stress difference N1, shows that the time required to reach steady state values is relatively short, probably in relation to the high degree of packing of these highly concentrated suspensions. However, N1 changes from positive to negative values with time, and finally reaches a negative steady state value. Moreover, it is shown that the second normal stress difference N2, compared to N1, takes significant steady state values, the sign of which depends on the applied shear rate. The steady shear stress (or viscosity) versus shear rate curve resembles in shape the three-region flow curve observed in a number of liquid crystalline polymers, the intermediate region (II) corresponding to the transition between two shear-thinning regions. The N1 versus shear rate flow curve shows a negative minimum, which takes place in the intermediate region. These flow curves are analyzed in terms of shear induced structural changes, on different length scales. Possible interplate friction resulting from lubricated contacts is also considered. © 2003 The Society of Rheology.